Early lactation in mothers of preterm newborns: a prospective longitudinal study of expressed milk volume and macronutrient composition during the first 15 days

Establishing breastfeeding in mothers of preterm infants is particularly challenging because direct breastfeeding is often not feasible during the first days after birth, making milk expression essential for initiating and maintaining lactation. Early expressed milk volume may predict subsequent lactation success, whereas the macronutrient composition of preterm human milk may vary according to maternal and neonatal factors. This study aimed to assess expressed milk volume and macronutrient composition during the first 15 postpartum days in mothers of preterm infants and to identify early predictors of adequate milk production. We conducted a prospective, longitudinal observational study with repeated measures over a 1-year period in a level III neonatal intensive care unit. Twenty-seven mothers of 34 preterm infants (≤ 32 weeks’ gestational age and/or birth weight ≤ 1,500 g) were included. Total expressed milk volume over 24 h was recorded on postpartum days 3, 4, 7, and 15. Fresh milk samples collected on days 3, 7, and 15 were analysed for protein, carbohydrate, and lipid content using Fourier-transform infrared spectroscopy. Total expressed milk volume increased significantly over time, from a median of 23 mL on postpartum day 3 to 505 mL on day 15 ( p < 0.001). Lactogenesis II occurred at a median of 4 days postpartum. Milk volume on postpartum day 4 was significantly associated with milk production at 2 weeks: all mothers expressing ≥ 140 mL/day on day 4 achieved ≥ 500 mL/day on day 15 ( p < 0.001). Initiation of milk expression within the first postpartum hour was associated with significantly higher milk volumes on days 4 and 7. Protein concentration decreased significantly over time, whereas carbohydrate and lipid concentrations increased. Protein content was higher in mothers aged ≥ 35 years on days 3 and 7 and was negatively correlated with gestational age and birth weight on day 15. Early monitoring of expressed milk volume, particularly on postpartum day 4, may help identify mothers of preterm infants at risk of inadequate milk production. Very early initiation of milk expression appears to support improved milk output, whereas changes in milk macronutrient composition highlight the potential value of individualised nutritional strategies for preterm infants in neonatal intensive care settings.